This invention relates to an improvement in a phosphor screen of a color picture tube.
As is well known, in the conventional color picture tubes, rare earth phosphors are used as red phosphor. For instance, red phosphor slurry for use in forming red dots or strips on the picture screen is prepared by mixing water, polyvinylalcohol, phosphor powder of Y.sub.2 O.sub.3 :Eu or Y.sub.2 O.sub.2 S:Eu together, to form a slurry of suitable viscosity, and then a photosensitive activating agent consisting of ammonium bichromate is added. The phosphor dots or strips are formed by applying the slurry on the picture screen, i.e., the inner face of the face plate, drying the slurry to form a film, exposing the film to photochemical rays through apertures of a shadowmask, and then developing the film to form dots or strips at the exposed places.
It is generally known that the europium-activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu) has high luminosity in red emission. However, the Y.sub.2 O.sub.3 :Eu phosphor has a shortcoming that it is liable to adverse hydrolysis when blended with the ammonium bichromate. By means of the hydrolysis, the photosensitivity for the photochemical rays of polyvinyl alcohol activated by the ammonium bichromate is considerably decreased. Therefore, the phosphor slurry changes to necessitate very long exposure time. Furthermore, chromium ions produced on the surface of the phosphor are not easily washed out by the subsequent developing, i.e., washing process after the exposure, and when heated in the subsequent process, are diffuse deep into the phosphor particle through, for instance, lattice imperfections. On account of this chromium-contamination, the phosphor is partly decomposed to change its characteristic and decreases luminosity. Furthermore, Y.sub.2 O.sub.3 :Eu phosphor is less hydrophilic, and therefore, is difficult to be sufficiently dispersed in the slurry, and is likely to produce specks on red phosphor emission. Though it is known that addition of a suitable Ca-compound and/or Ba-compound in a manner to coat the phosphor particle is useful in alleviating the hydrolysis, such art is not satisfactory in eliminating the shortcoming.
Another known method for alleviating the shortcoming is to add water glass as a dispersing agent, but the employment of the water glass causes gel formation and damages characteristic of the phosphor slurry.
On the other hand, Y.sub.2 O.sub.2 S:Eu phosphor does not have the abovementioned shortcomings like of Y.sub.2 O.sub.3 :Eu phosphor, but has insufficient luminosity.